Four Coloradans are among 44 former U.S. senators who signed a letter in The Washington Post saying the Senate must stand in “defense of our democracy” as the nation enters “a dangerous period.”

“As former members of the U.S. Senate, Democrats and Republicans, it is our shared view that we are entering a dangerous period, and we feel an obligation to speak up about serious challenges to the rule of law, the Constitution, our governing institutions and our national security,” the letter says.

Among the ex-senators who signed the letter, posted by The Post on Monday night, are Colorado Republican Ben Nighthorse Campbell and Democrats Gary Hart, Mark Udall and Tim Wirth.

The letter mentions the approaching conclusion of special counsel Robert S. Mueller III’s investigation “and the House’s commencement of investigations of the president and his administration. The likely convergence of these two events will occur at a time when simmering regional conflicts and global power confrontations continue to threaten our security, economy and geopolitical stability.”

Mueller has been probing Russian interference in the 2016 election as well as any collusion between the Trump campaign and Russian representatives. Prosecutors also appear to be investigating possible campaign finance violations.

President Donald Trump has repeatedly denied there was any collusion between his campaign and Russia, and refers to the Mueller probe as a “witch hunt.” Some lawmakers have expressed concern that Trump will try to block Mueller’s probe or remove him.

The former senators’ letter alludes to signals from Democrats who will control the U.S. House of Representatives next year that they will launch investigations of the Trump administration.

U.S. Rep. Jerry Nadler, D-N.Y., who is likely to be the next chair of the House Judiciary Committee, told CNN in November that the Trump administration “is going to be held accountable” in the next Congress.

Nadler said over the weekend that it would be an “impeachable offense” if it’s proved that Trump directed hush-money payments during the 2016 campaign to women who say he had affairs with them.

The House can impeach a president with a simple-majority vote, as it did during Bill Clinton’s presidency. It then would take a two-thirds vote by the Senate to remove a president following an impeachment trial.

The letter adds:

“We are at an inflection point in which the foundational principles of our democracy and our national security interests are at stake, and the rule of law and the ability of our institutions to function freely and independently must be upheld.”

In all, 32 Democrats signed the letter, 10 Republicans and two independents.

It concludes:

“Regardless of party affiliation, ideological leanings or geography, as former members of this great body, we urge current and future senators to be steadfast and zealous guardians of our democracy by ensuring that partisanship or self-interest not replace national interest.”

Here is the full text of the letter and its signatories:

We are former senators. The Senate has long stood in defense of democracy – and must again.

Dear Senate colleagues,

As former members of the U.S. Senate, Democrats and Republicans, it is our shared view that we are entering a dangerous period, and we feel an obligation to speak up about serious challenges to the rule of law, the Constitution, our governing institutions and our national security.

We are on the eve of the conclusion of special counsel Robert S. Mueller III’s investigation and the House’s commencement of investigations of the president and his administration. The likely convergence of these two events will occur at a time when simmering regional conflicts and global power confrontations continue to threaten our security, economy and geopolitical stability.

It is a time, like other critical junctures in our history, when our nation must engage at every level with strategic precision and the hand of both the president and the Senate.

We are at an inflection point in which the foundational principles of our democracy and our national security interests are at stake, and the rule of law and the ability of our institutions to function freely and independently must be upheld.

During our service in the Senate, at times we were allies and at other times opponents, but never enemies. We all took an oath swearing allegiance to the Constitution. Whatever united or divided us, we did not veer from our unwavering and shared commitment to placing our country, democracy and national interest above all else.

At other critical moments in our history, when constitutional crises have threatened our foundations, it has been the Senate that has stood in defense of our democracy. Today is once again such a time.

Regardless of party affiliation, ideological leanings or geography, as former members of this great body, we urge current and future senators to be steadfast and zealous guardians of our democracy by ensuring that partisanship or self-interest not replace national interest.

Max Baucus (D-Mont.), Evan Bayh (D-Ind.), Jeff Bingaman (D-N.M.), Bill Bradley (D-N.J.), Richard Bryan (D-Nev.), Ben Nighthorse Campbell (R-Colo.), Max Cleland (D-Ga.), William Cohen (R-Maine), Kent Conrad (D-N.D.), Al D’Amato (R-N.Y.), John C. Danforth (R-Mo.), Tom Daschle (D-S.D.), Dennis DeConcini (D-Ariz.), Chris Dodd (D-Conn.), Byron Dorgan (D-N.D.), David Durenberger (R-Minn.), Russ Feingold (D-Wis.), Wyche Fowler (D-Ga.), Bob Graham (D-Fla.), Chuck Hagel (R-Neb.), Tom Harkin (D-Iowa), Gary Hart (D-Colo.), Bennett Johnston (D-La.), Bob Kerrey (D-Neb.), John Kerry (D-Mass.), Paul Kirk (D-Mass.), Mary Landrieu (D-La.), Joe Lieberman (I-Conn.), Blanche Lincoln (D-Ark.), Richard Lugar (R-Ind.), Barbara Mikulski (D-Md.), Ben Nelson (D-Neb.), Sam Nunn (D-Ga.), Larry Pressler (R-S.D.), David Pryor (D-Ark.), Don Riegle (D-Mich.), Chuck Robb (D-Va.), Jay Rockefeller (D-W.Va.), Jim Sasser (D-Tenn.), Alan Simpson (R-Wyo.), Mark Udall (D-Colo.), John W. Warner (R-Va.), Lowell Weicker (I-Conn.), Tim Wirth (D-Colo.)